"Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose," a representative for Mr Sistani told followers during Friday prayers in the holy city of Kerbala.

Those killed fighting ISIS militants would be martyrs, he said as worshippers chanted in acknowledgement.

A major offensive, spearheaded by the jihadist ISIS, but also involving supporters of now-executed dictator Saddam Hussein, has overrun a large chunk of northern and north-central Iraq since Monday.

Security forces performed poorly, with some abandoning their vehicles and positions and discarding their uniforms.

Witnesses said on Friday that militants were gathering in the Samarra area for another attack on the city, after being repulsed earlier in the week.

Iran to 'consider' joint role with US against ISIS militants in Iraq

Iran's president, Hassan Rouhani, says his country stands ready to help Iraq's government in its fight against ISIS insurgents within the framework of international law, although Baghdad has so far not requested assistance.

Shiite Muslim Iran, which has strong leverage in Shiite-majority Iraq, is so alarmed by the Sunni jihadist advance from Iraq's north that it may be ready to cooperate with longtime arch-enemy Washington in helping Baghdad fight back.

"We all should practically and verbally confront terrorist groups," Mr Rouhani told a press conference broadcast live on state television.

Asked if Tehran would work with the United States in tackling insurgents in Iraq, he replied: "We can think about if we see America starts confronting the terrorist groups in Iraq or elsewhere."

United States Defense Secretary Chuck Hagel has ordered an aircraft carrier move into the Gulf, in case Washington decides to pursue a military option.

"The order will provide the Commander-in-Chief additional flexibility should military options be required to protect American lives, citizens and interests in Iraq," the Pentagon said in a statement on Saturday.

The carrier USS George H.W. Bush, moving from the North Arabian Sea, will be accompanied by the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun.

On Friday, US President Barack Obama said he needed several days to determine how the US would help Iraq deal with the insurgency.

"The United States is not simply going to involve itself in a military action in the absence of a political plan by the Iraqis that gives us some assurance that they are prepared to work together," he said.

Speaking at the White House, Mr Obama said the threat by militants from ISIS in Iraq poses a danger to the people of Iraq and also potentially Americans.

"This is a regional problem, and it is going to be a long-term problem," he said.

"What we're going to have to do is combine selective actions by our military to make sure that we're going after terrorists who could harm our personnel overseas or eventually hit the homeland."

Meanwhile, Britain announced it would provide $5 million of emergency humanitarian assistance to help civilians fleeing the jihadists who have overrun a large chunk of northern Iraq.

The initial support package includes medicine, clean water and sanitation equipment, basic household items and hygiene kits for women.

It also includes "protection for vulnerable girls and women through the deployment of dedicated UN safety and welfare teams" in camps, the Department for International Development said in a statement.